The present invention relates, in general terms to automatic liquid rheostats appropriate for starting any type of motor, and more particularly concerns liquid rheostats which are of variable level.
Such a liquid rheostat, which is described, for example, in French Pat. No. 1,216,101, comprises, in general terms, a reservoir which contains the electrolyte, a starting cell, in the upper part of which are located electrodes, and a connecting pipeline, which is suitable for the controlled passage of electrolyte between the said reservoir and the said cell.
Where, for example, the starting of a motor with rotor starting is concerned, the electrodes of such a liquid rheostat are connected to the winding of such a motor, and control means are provided to ensure, for starting, that the electrolyte progressively enters the starting cell by forced circulation of this electrolyte from the reservoir to the starting cell through the connecting pipeline.
During starting, an increased length of each electrode is progressively, immersed in the electrolyte, and thus short-circuited, and only the parts of the electrodes which project from the electrolyte continue to introduce resistance into the circuit.
In order that the starting is as gradual as possible, it is important that the area of the surface of each electrode in contact with the electrolyte can be greatly varied, the contact surface area being at a minimum at the commencement of starting and at a maximum on termination of starting.
In the liquid rheostats known hitherto, either the electrodes are solid, such being the case in German patent application No. 1,465,596, or the electrolytes are hollow but each constitute a closed volume, such being the case in the abovementioned French Pat. No. 1,216,101; in all cases, only the external periphery of the electrodes is in contact with the electrolyte.
As a result, the maximum surface area of such an electrode in contact with the electrolyte is necessarily small.